Investigators suspect that the mole – not yet identified – gave the Swiss spy the names of tax investigators, and that the Swiss Federal Intelligence Service (FIS) ran the operation.

Since 2010 North Rhine-Westphalia (NRW), Germany’s most populous state, has paid whistleblowers millions of euros for data CDs revealing secret Swiss bank accounts. They are trying to recover millions of euros hidden by German tax dodgers.

‘Spy thriller’

It is thought the alleged Swiss spy had a list of German investigators, provided by the FIS, and used his secret ministerial informer to verify those names.

That intelligence could then be used to accuse specific German officials of violating Swiss banking laws and engaging in commercial espionage.

“The scandal reaches new proportions when spies sign up informers in the finance administration, in order to spy on successful NRW tax investigators and play into the hands of people who make billions in profit at the expense of society,” he said.

“It’s hard to believe that such a spy thriller took place not on the screen but on our own doorstep.”

The German federal government has asked Switzerland for an explanation concerning the alleged spying.

Image copyrightAFPImage caption Swiss francs: The country’s banks have long profited from their “safe haven” reputation

The arrest warrant for the 54-year-old, named only as Daniel M, was issued in December.

The warrant, cited by German media, alleges that the acting FIS chief was in mobile phone contact with Daniel M. The FIS is said to have paid €90,000 (£76,300; $98,380) for planting the mole.

Reports say the alleged spy was also helped by a German ex-police officer, who is now a security consultant.

In recent years the Swiss government has moved to make its banks more transparent. It signed a tax transparency agreement with the EU in 2015.